Drilling mud is known to represent a heterogeneous liquid system necessarily containing colloidal solid phase particles. The presence of such particles in drilling mud determines a number of rheological properties of the drilling mud which are important from the point of view of well drilling quality. Drilling mud should retain these properties which enable the best well drilling conditions. The maintenance of properties of drilling mud during drilling is, however, a very difficult problem.
The majority of drilling operations are conducted in clayey rocks. The clayey rock being drilled is partly dispersed into highly colloidal particles admixed to the drilling mud so that after several cycles of pumping of the drilling mud through the well during drilling the composition of drilling mud is undesirably changed.
Restoration of drilling mud properties makes it necessary to use various methods for improving quality of drilling mud. Therefore, an efficient cleaning of drilling mud from drillings is of a predominant importance for the well drilling process.
Technological and economical performance of drilling largely depends on quality of drilling mud being used and the degree of its cleaning from drillings.
High-grade cleaning of drilling mud improves the mechanical drilling speed and the conditions for operation of drill bits and other equipment. In addition to an improvement of mechanical speed of drilling, high-grade cleaning of drilling mud contributes to a reduction of consumption of materials used for maintaining properties of drilling mud, prolonging its service life and reducing complications and emergency situations during drilling.
Low quality of cleaning of drilling mud is the main cause of emergency situations and complications associated with loss of drilling mud, jamming of drilling and casing pipes, avalanches and outbursts of rock from the well walls.
Therefore, high-grade cleaning of drilling mud from drillings is the most important production process during well drilling which substantially affects the technological and economical performance of well drilling.
All existing apparatus (vibratory screens, hydrocyclones) for cleaning drilling mud (cf., Driller's Handbook by V. I. Mischevich, vol. I, 1973, "Nedra" Publishers, Moscow, pp. 369-375) (in Russian) enable the elimination of a certain part of solid particles from drilling mud at various rates and degrees. Thus, the smallest size of particles that can be separated from drilling mud on vibratory screens depends on the mesh size of the screen. With a reduction of mesh size, to improve quality of cleaning, the throughput capacity materially decreases and drilling mud losses with the waste increase.
In cleaning in hydrocyclones heavy-weight particles are removed from drilling mud. Fine particles (less than 20 .mu.m) appearing upon dispersion of drillings in drilling mud cannot be removed by hydrocyclones and other cleaning devices.
In addition, one of the most important problems in cleaning drilling mud in prior art apparatus is the problem of separation of drillings from drilling mud. Drillings are generally separated from drilling mud after the apparatus is stopped, and this is associated with large losses of drilling mud. This problem may be solved by cutting most compacted mass of the drillings off the main mass of drilling mud as the drillings are compacted. The most promising way of bringing a solution to this problem appears to be provided by an apparatus for cleaning drilling mud from drillings, comprising a straight-flow hopper having a discharge pipe and a device for removing drillings.
The device for removing drillings in a prior art apparatus comprises a conveyor screw and a valve cutting the interior of the hopper off the pipe (cf., USSR Inventor's Certificate No. 572279, cl. BO1D, 21/00, issued in 1976).
The prior art apparatus enables the removal of drillings deposited in the pipe together with a part (up to 30%) of drilling mud, so that a substantial quantity of drilling mud is lost thus resulting in added cost.